Air knife type systems conventionally comprise two blower nozzles arranged respectively on either side of the strip to be galvanized, in the area where the strip emerges from the galvanization bath. These two air knives are blown onto each side of the strip. These nozzles extend over the whole width of the strip, the plane of the air knives being substantially perpendicular to the main plane of the strip. The thickness of the zinc deposit obtained depends on the force of impact of the air knives on the strip which can be adjusted by acting on the pressure of the air in the nozzle or on the nozzle-to-strip distance.
This kind of nozzle is described, for example, in FR-A-2 090.313 and generally comprises an air supply and distribution chamber which opens into an acceleration chamber, delimited by two jaws which converge up to an opening delimited by two opposing lips, narrowly spaced so as to form an air knife blown in the direction of the strip.
In this arrangement, one of the jaws is fixed and the other deformable by the action of a plurality of push rods distributed over the whole of its length in order to vary the thickness of the fluid knife from its edges up to the center of the metal strip. This push rod is associated with an adjustment device individually actuated by a rod extending above the zinc bath and therefore subjected to the heat radiated by the zinc bath. Such an adjustment method is therefore not very accurate and difficult to control remotely.
In addition, to vary the thickness of the air knife, the moving jaw, whose base is embedded, on the side opposite the opening, on the wall of the distribution chamber, has a taper or slot which allows it to deform by pivoting about this slot in order to adjust the width of the air knife. As a result, the profile of the air knife, in cross-section, will become more or less flared as the thickness of the air knife is adjusted, and cause the pressure of the air jet to vary on the strip.
In another arrangement, described in document US-A-3.841.557, the width of the opening is adjusted by means of a series of electric elements placed inside one of the jaws and distributed over its entire length, the jaw also having a cooling system. The width of the slot is adjusted by a bimetallic-type action and can therefore be remotely controlled, but its range of adjustment is necessarily limited. Moreover, such an arrangement is fairly complicated and costly.